1. Field
Embodiments of the present invention generally to the field of semiconductor processing. More specifically, embodiments of the invention are directed to methods for improving substrate uniformity across a surface of the substrate during thermal processing.
2. Description of the Related Art
Rapid thermal processing (RTP) is a process for annealing substrates during semiconductor processing. During RTP, thermal radiation is generally used to rapidly heat a substrate in a controlled environment to a high temperature such as about 1200° C. This high temperature is maintained for a specific amount of time ranging from less than one second to several minutes depending upon the process. The substrate is then cooled to room temperature for further processing.
RTP usually requires a substantially uniform temperature profile across the substrate. Temperature uniformity in the substrate is necessary to prevent thermal stress-induced substrate damage such as warpage, defect generation and slip. Particularly given the submicron dimensions of current devices, to obtain high yields and process reliability, the temperature of the substrate must be precisely controlled during these thermal processing steps. For example, to fabricate a dielectric layer 60-80 Å thick with a uniformity of ±2 Å, the temperature in successive processing runs cannot vary by more than a few degrees Celsius from the target temperature. To achieve this level of temperature control, the temperature of the substrate is typically measured in real time and in situ.
Optical pyrometry is a technology that is used to measure substrate temperatures in RTP systems. An optical pyrometer using an optical probe samples the emitted radiation intensity from the substrate, and computes the temperature of the substrate based on the spectral emissivity of the substrate and the ideal blackbody radiation-temperature relationship. A computerized controller receives the outputs of the pyrometers and accordingly controls the voltages supplied to the lamps in different zones to thereby tailor the radial distribution of radiant energy. One difficulty encountered in the use of pyrometers for measuring substrate temperature in an RTP system is that variations between individual temperature sensors and differences in their position with respect to a particular substrate in the chamber can affect the accuracy of the temperature measurements. Therefore, substrate temperature measurements obtained from the sensors can have an unknown error component attributable to such variations. Those variations show up, for example, as differences in the thickness of a deposited layer across the substrate surface because the sensors are used as part of the closed-loop temperature control.
One way to address errors in temperature measurements is to reduce or increase a temperature offset at the location of the temperature deviation. Such a technique assumes localized heating from the heat source. However, due to cross-coupling between lamp zones, thermal discontinuities at the substrate edge, and the viewing angle of the probes, such an assumption generally is not valid. Thus, additional techniques are required for obtaining accurate substrate temperature measurements to provide uniform processing conditions across the substrate surface.
Therefore, there is a need for methods used in RTP for improved substrate uniformity.